where AP is the pressure differential due to capillary action, AP,,,,, is the pressure drop in the vapor region and AP is the liquid pressure drop experienced in the wick. Thus, for effective heat pipe operation, the pressure differential due to capillary action, AP must be equal to or exceed the sum of the vapor and liquid pressure drops experienced in the vapor region and the wick respectively. The greater the difference, the

greater the heat transfer capability of the heat pipe. If

the wick were totally manufactured from fine pore material, the liquid friction flow factor, AP would be excessively large due to the restrictions to flow.

The prior art has retained the aforementioned critical properties by fabricating channels into the heat pipe walls by a broaching process. The channels, which permit unrestricted fluid flow from the heat pipe condenser to the evaporator section, are covered by a fine mesh screen to establish greater capillary wicking forces. Composite wicks have also been fabricated by placing layers of heavy mesh screen to 60 mesh) beneath the liquid/vapor interface layers of fine mesh screen (200 to 400 mesh). Another technique comprises the fabrication of open annulus wicks by swaging several turns of screen wound between two copper tubes. The copper tubes are then etched away and the porous rigid wick is sinter bonded. Upon insertion into a heat pipe with an open annulus between the wick and the heat pipe walls, this wick presents an optimum arrangement for liquid metal charged heat pipes. The first two of the aforementioned techniques have the disadvantages of being both uneconomical and time consuming and the latter technique is only suitable for liquid metal working fluids, since a low thermal conductivity fluid would boil beneath the capillary drawing free wick. Although other wick structures have been fabricated, the three mentioned above are the ones most frequently employed.

SUMMARY OF THE INVENTION Briefly, this fabrication technique comprises winding several turns of fine mesh screen onto a cylindrical mandrel. Fine wires are then positioned axially along the mandrel over the fine mesh screen. Additional layers of fine mesh screen are wound over the assembly and it is inserted into an outer tube. The final assembly is swaged and the mandrel, wires and outer tube are dissolved away.

This process may also be used to replace the costly broaching process used to fabricate channels in a heat pipe container tube. A similar wick can be fabricated by utilizing a fine mesh screen/mandrel assembly which is covered with axial wires and inserted into an outer tube and swaged. Dissolving away the mandrel and wires leaves a tight homogeneous, channeled wick.

Thus, the present fabrication technique provides fine pore sizes at the wick liquid/vapor interface and channels immediately below this interface for unrestricted fluid flow (low friction flow). The resulting process is economical and produces a wick that can be employed with either high or low thermal conductivity fluids.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of an exemplary embodiment of this invention, reference may be had to the accompanying drawings, in which:

FIG. 1 is an isometric view of a heat pipe, having a portion thereof cut away for clarity, and including an enlarged longitudinal sectional view of a portion of a heat pipe wick constructed in accordance with the teachings of this invention;

FIG. 2 is an isometric view of a heat pipe wick assembly broken away in layers, and is illustrative of an interim stage of the wick assembly during fabrication of the heat pipe wick of FIG. 1;

FIG. 3 is a cross-sectional view of the heat pipe wick assembly of FIG. 2 and is taken along the lines III-III thereof;

FIG. 4 is a cross-sectional view of the heat pipe wick of FIG. 1;

FIG. 5 is an isometric view of a heat pipe wick assembly broken away in layers and is illustrative of an interim stage of the wick assembly during fabrication of another embodiment of a heat pipe wick constructed in accordance with the teachings of this invention; and

FIG. 6 is a cross-sectional view of another embodiment of a heat pipe wick constructed in accordance with the teachings of this invention and is illustrative of the final wick derived from the interim assembly illustrated in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the heat pipe illustrated in FIG. I, it will be appreciated that a heat pipe 10 constructed in accordance with the principles of this invention includes an evacuated chamber 12 whose inside walls are lined with a capillary structure, or wick 30, that is saturated with a volatile fluid. The operation of a heat pipe combines two familiar principles of physics; vapor heat transfer and capillary action. Vapor heat transfer serves to transport the heat energy from the evaporator section 14 at one end of the pipe to the condenser section 16 at the other end. Capillary action returns the condensed working fluid back to the evaporator section 14 to complete the cycle.

The working fluid absorbs heat at the evaporator section 14 and changes from its liquid state to a gaseous state. The amount of heat necessary to cause this change of state is the latent heat of vaporization. As the working fluid vaporizes, the pressure at the evaporator section 14 increases. The vapor pressure sets up a pressure differential between the ends of the heat pipe, and this pressure differential causes the vapor, and thus the heat energy, to move towards the condenser section 16. When the vapor arrives at the condenser section 16, it

is subjected to a temperature slightly lower than that of the evaporator section 14 and condenses, thereby releasing the thermal energy stored in its heat of vaporization at the condenser section 16 of the heat pipe. As the vapor condenses the pressure at the condenser section 16 decreases so that the necessary pressure differential for continued vapor heat flow is maintained.

Movement of the fluid from the condenser section 16 back to the evaporator section 14 is accomplished by capillary action within the wick 30 which connects the condenser 16 to the evaporator 14.

As is known, the driving force that causes the liquid to move through the capillary is the surface tension of the liquid. When a fluid is placed in a compatible vessel, that is a vessel composed of a material that the fluid wets well, there is an attractive force between the fluid and the walls of the vessel. This force combines with the surface tension in such a way as to move the liquid towards the unfilled portion of the vessel. If the vessel is a capillary of small diameter, such as the wick 30, this force, called capillary attraction, can be large compared with the mass of fluid in the capillary. The resulting forces will thus cause the liquid to pump itself through the wick indefinitely in the absence of other forces. It is the use of vapor pressure and capillary action that enables the heat pipe to operate as a self-contained heat pump.

For liquid metal working fluids the wick 30 must retain several critical properties. For example the wick liquid/vapor interface 32 must possess extremely small pore sizes for optimum capillary drawing forces. This can be seen from the following equation;

AP Z AP AP where AP, is the pressure differential due to capillary action, AP is the pressure drop in the vapor region and AP, is the liquid pressure drop experienced in the wick. Thus, for effective heat pipe operation, the pressure differential due to capillary action, AP must equal or exceed the sum of the vapor, AP,,,,,,, and liquid, AP pressure drops experienced in the vapor region and wick respectively. If the wick were totally manufactured from fine pore material, the liquid friction flow factor, A t" would be excessively large due to the restrictions to flow. A more detailed explanation of heat pipe operation is presented in the May, 1968 issue of Scientific American, in an article entitled 'Ihe Heat Pipe, by Y. Eastman.

Referring now to the heat pipe wick assembly illustrated in FIGS. 2 and 3, it will be appreciated that a heat pipe wick assembly 35 constructed in accordance with the principles of this invention includes several turns of fine mesh screen 36, from approximately 200 to 500 mesh (or higher mesh), desirably constructed from a material that is relatively resistant to being etched away in the chosen etchant specified below, such as 304 stainless steel or pressed and sintered felt metal, which are wound onto a central mandrel 38,

desirably constructed from a material that can be etched away in the chosen etchant. The mandrel 38 may be designed so as to assume any desired shape depending upon the desired shape of the wick being fabricated, but it is to be understood that the shape of the mandrel is not to be limited to the hollow circular cylindrical configuration illustrated by reference character 38. Fine wires 40, desirably constructed from a material that will dissolve in the chosen etchant, are laid axially along the mandrel 38 over the fine mesh screen 36. The size of the wires will depend upon the working fluid being employed in the heat pipe. For example, for a working fluid such as sodium the diameter of the wires may vary approximately from 15 to 25 mills. Additional layers of fine mesh screen 42, approximately from 200 to 400 mesh, desirably formed from the same material as the mesh screen 36, are wound over the assembly 35 and inserted into an outer tube 44, which is constructed from a material having the same characteristics as that of the central mandrel 38, and which closely receives the assembly 35 in the center thereof. The assembly is then shaped by swaging or by any other process that uniformly constricts the interface between the two mesh layers 36 and 42 so that they closely conform to the axially laid wire surface at the wire/mesh interface and form a continuous mesh cross-section at the mesh/mesh interface. Such a process may also be accomplished by expanding the central mandrel 38 against the fixed outer tube 44 by internally pressurizing and/or heating the central mandrel 38. The mandrel 38, wires 40 and outer tube 44 are then dissolved away in a suitable etching solution such as nitric acid where the components to be etched away are constructed from copper, or sodium hydroxide may be used where the components to be etched away are constructed from aluminum. It is to be understood that any other etchant may be used that will suitably react with the components to be dissolved away without dissolving the mesh screen.

Referring now to FIG. 4, it will be observed that a free standing wick 46 with channels 48 just beneath the evaporator surface 50 results from this process. It can also be seen that this process can easily replace the costly breaching process previously used to fabricate channels in a heat pipe container tube. This may be accomplished by the assembly illustrated in FIG. 5, which is similar to the assembly illustrated in FIG. 2. This assembly includes several turns of fine mesh screen 52, formed from a material having the same characteristics as the mesh screen 36, which are wound around a central mandrel 54, having the same characteristics as the mandrel 38. Fine wires 56, which are formed from a material having the same characteristics as that of the wires 40, are laid axially along the mandrel 54 over the fine mesh screen 52. The assembly is then inserted into an outer tube 58, constructed from a material that is relatively resistant to the chosen etchant mentioned below. The entire assembly is then swaged, or by any other process uniformly constricted, so that the mesh screen 52 conforms to the shape of the axially laid wires 56 and outer tube 58 at the wire/mesh and tube/mesh interface respectively. The mandrel 54, and wires 56 are then dissolved away in a suitable etching solution such as nitric acid where the components to be etched away are constructed from copper, or sodium hydroxide where the components to be etched away are constructed from aluminum. It is to be understood that any other etchant may be used that will suitably react with the components to be dissolved away without dissolving the mesh screen 52 and outer tube 58. Referring now to FIG. 6 it will be observed that the resulting structure is a tight homogeneous, channeled wick similar to the wick fabricated by the aforementioned broaching process.

We claim:

1. A process for fabricating a heat pipe wick having channels just beneath the fine pore surfaces, which comprises winding several turns of fine mesh screen over a mandrel, placing a plurality of fine wires axially along said mandrel over said fine mesh screen and inserting the resulting assembly into an outer tube which closely receives said assembly in the center thereof, constricting the final assembly so that the mesh screen conforms to the shape of the axially laid wires and outer tube at the wire/mesh and tube/mesh interface respectively and then dissolving away said mandrel and said wires.

2. The process for fabricating a heat pipe wick of claim 1 wherein said mandrel and said wires are formed from copper and said fine mesh material and said outer tube are chosen to be insoluble in nitric acid.

3. The process for fabricating a heat pipe wick of claim 2 wherein said dissolving step comprises etching away said copper wires and said copper mandrel in nitric acid.

4. The process for fabricating a heat pipe wick of claim 1 wherein said mandrel and said wires are formed from aluminum and said fine mesh material and said outer tube are chosen to be insoluble in sodium hydroxide.

5. The process for fabricating a heat pipe wick of claim 4 wherein said dissolving step comprises etching away said aluminum wires and said aluminum mandrel in sodium hydroxide.

6. The process for fabricating a heat pipe wick of claim 1 wherein said fine mesh screen is constructed from a material selected from the group consisting of 304 stainless steel and pressed and sintered felt metal.

7. The process for fabricating a heat pipe wick of claim 1 wherein said constricting step comprises swaging said final assembly.

8. The process for fabricating a heat pipe wick of claim 1 wherein said constricting step comprises expanding said mandrel against said rigid outer tube.

9. The process for fabricating a heat pipe wick of claim 1 including winding several turns of fine mesh screen over said axially laid wires, constricting said final amembly so that the mesh screen closely conforms to the axially laid wire surface at the wire/mesh interface and forms a continuous mesh cross-section at the mesh/mesh interface and dissolving away said outer tube concurrently with said wires and said mandrel.

10. The process for fabricating a heat pipe wick of claim 9 wherein said mandrel, said wires and said outer tube are formed from copper and said fine mesh material is chosen to be insoluble in nitric acid.

12. The process for fabricating a heat pipe wick of claim 9 where said mandrel, said wires and said outer tube are formed from aluminum and said fine mesh material is chosen to be insoluble in sodium hydroxide.

13. The process for fabricating a heat pipe wick of claim 12 wherein said dissolving step comprises etching away said aluminum wires, said aluminum mandrel and said aluminum outer tube in sodium hydroxide.

14. The process for fabricating a heat pipe wick of claim 9 wherein said fine mesh screen is constructed from a material selected from the group consisting of 304 stainless steel and pressed and sintered felt metal.